1. Field of the Invention
The present invention relates to a three-phase inverter system that converts a DC voltage to three-phase AC voltages to supply the AC voltage to a pulsating load such as an induction motor that causes ripples and the control method of the system.
2. Background Art
In a three-phase inverter system for a rolling stock in a DC feeding system, for example, a DC voltage inputted from a pantograph is converted into a three-phase AC voltage by the operation of an inverter section and is supplied to a compressor motor as a driving source of a breaking system, a door driving system and an air-conditioning facility and to a resistive load such as a fluorescent lamp.
Here, a compressor motor has the property of causing an input current to periodically pulsate depending on the number of the cycle of compressing air. This presents a problem in that voltage drops are periodically produced on the input side of the compressor motor due to the presence of a circuit element such as a filter to thereby cause voltage fluctuation to make fluorescent lamps flicker which are connected to the same feed line.
Against this, in JP-A-4-340369 (paragraphs [0015] to [0030] and FIG. 1 etc.), a technology (a first related art) is disclosed which suppresses voltage fluctuations produced when a compressor motor is driven to prevent fluorescent lamps from flickering in a three-phase inverter system for a rolling stock.
FIG. 5 is a diagram showing the whole configuration of a three-phase inverter system according to the first related art.
In FIG. 5, the output of a DC-to-DC converter 103 being connected between a pantograph 101 and a wheel 102 is inputted to a three-phase inverter system 200.
The three-phase inverter system 200 is provided with an inverter bridge 201, a waveform shaping circuit formed of AC reactors 202 and capacitors 203 connected onto the AC output side of the inverter bridge 201, a transformer 204 connected onto the AC output side of the inverter bridge 201 and a diode bridge 205 connected onto the output side of the transformer 204. The three-phase inverter system 200 is further provided with an inverter control circuit 220 and a driving circuit 206 that drives the inverter bridge 201 by the output of the inverter control circuit 220.
The inverter control circuit 220 is provided with a reference voltage generator 221, a lag element of first order 222 that provides a time-lag of first order to the difference between the output voltage value of the diode bridge 205 and the reference voltage value and a PWM (Pulse Width Modulation) converter 223.
On the output side of the three-phase inverter system 200, in addition to the foregoing, a load fluctuation compensating circuit 210 is provided. The load fluctuation compensating circuit 210 is provided with current transformers 211 detecting the AC outputs of the three-phase inverter system 200, a diode bridge 212 connected to the current transformers 211, a band-pass filter 213 connected onto the output side of the diode bridge 212, an amplifier 214 and a phase-shift circuit 215. The phase-shift circuit 215 is connected to the inverter control circuit 220 so that the output of the phase-shift circuit 215 is added to the output of the lag element of first order 222 in the inverter control circuit 220.
The output of the three-phase inverter system 200 outputted from the load fluctuation compensating circuit 210 is supplied to a compressor motor (an induction motor) M.
In the load fluctuation compensating circuit 210, AC output currents detected by the current transformers 211 are converted into a DC voltage containing the foregoing voltage fluctuation as the component thereof through the diode bridge 212. From thus converted DC voltage, only the fluctuating component therein is extracted with ripples with the frequency of the power supply eliminated by the band-pass filter 213. The fluctuating component is thereafter amplified by the amplifier 214. The output voltage of the amplifier 214 is then inputted to the phase-shift circuit 215 in which the phase of the voltage is shifted by 180 degrees, by which a compensation signal is produced for cancelling the fluctuating component. The compensation signal is inputted to the inverter control circuit 220 to thereby execute interruption to the voltage control.
In the inverter control circuit 220, the difference between the value of the reference voltage from the reference voltage generator 221 and the detected value of the output voltage from the diode bridge 205 is inputted to the lag element of first order 222, in which the arithmetic operation on the conduction ratio for PWM control is carried out. To the operated conduction rate, the value of the compensation signal is added, by which the conduction ratio is corrected and the corrected conduction ratio is inputted to the PWM converter 223 at which the inputted conduction ratio is converted into turning-on and -off signals for the semiconductor switching elements in the inverter bridge 201. On the basis of the turning-on and -off signals, the semiconductor switching elements are made to be turned-on and -off through the driving circuit 206, by which fluctuations in the output voltages of the three-phase inverter system 200 due to the operation of the compressor motor M are suppressed to prevent fluorescent lamps from flickering.
In addition, in JP-A-2006-325326 (paragraph [0010] etc.), a three-phase inverter system (a second related art) is described with an object of stabilizing an output voltages like the three-phase inverter system disclosed in JP-A-4-340369.
In the second related art, as is shown in FIG. 6 as a diagram showing the configuration of the principal part of the three-phase inverter system according to the second related art, a reactive current iQ including a fluctuating component is detected on the output side of the three-phase inverter system and the difference between the value of the detected reactive current iQ and the value of a reactive current, which is provided by making the detected reactive current iQ pass through a low-pass filter 301 to be averaged, is obtained. The difference is then multiplied by a proportional gain 302 (K) and an output Δf as the result of the multiplication by the proportional gain 302 is added to a reference frequency f for obtaining the final frequency command f* of the three-phase inverter system, by which the output voltage of the system is made to be stabilized.
In the three-phase inverter system according to the first related art shown in FIG. 5, when the effect of suppressing low frequency fluctuating components in the output voltages was verified by using an induction motor as a load and using the band-pass filter 213 with the center frequency thereof at a power frequency (50 Hz or 60 Hz), it was shown that even though the compensation signal from the load fluctuation compensating circuit 210 was used, the frequency of the compensation signal had a large deviation from the actual low frequency fluctuating component to sometimes make it impossible to sufficiently suppress the fluctuations in the output voltages.
In addition, compensation carried out by extracting only the fluctuating component of an output current therefrom causes an excessively small or large effect of suppression on some average values of root mean square values of output currents even though the fluctuation widths of low frequency current components thereof are equal to make it impossible to obtain a proper effect of suppression.
Furthermore, addition of a compensation signal to the difference between the value of the reference voltage and the detected value of the output voltage in the three-phase inverter system is equivalent to introducing disturbance to the value of an output voltage command. Therefore, when the gain of the compensation signal is high, the output voltages of the three-phase inverter system became sometimes impossible to be kept constant.
Moreover, in the three-phase inverter system according to the second related art shown in FIG. 6, only by compensating the frequency command with the use of the compensation signal proportional to the difference between the reactive current and the reactive current after passing through the low-pass filter, it was difficult to sufficiently suppress the fluctuations in the output voltages of the three-phase inverter system.
Accordingly, it is an object of the present invention to provide a three-phase inverter system and the control method thereof which can suppress fluctuations in the voltages and currents which are inputted and outputted better than the related ones did.